JOURNAL OF CHEMICAL EDUCATION
A "CLOCK REACTION*' FOR A BEGINNING 'COURSE I N COLLEGE CHEMISTRY ARTHUR H. BLACK and VANCE H. DODSON University of Toledo, Toledo, Ohio
A
NUMBER of "clock reactions" designed for either of the reaction. If aqueous sulfuric acid solutions of lecture demonstration or student experimentation in ceric sulfate (or ammonium sulfatocerate) of a given general chemistry have been described in THIS JOUR-molarity are reacted with various concentrations of sodium oxalate (also in aqueous sulfuric acid), the time N A L . ' - ~ Many of these proposed reactions have the disadvantage of requiring a rather complicated explana- required for decolorization should be a linear function tion of the mechanism in order to relate the measured of t,he oxalate ion concentration. The concentration time with the chemical equation for the reaction. of the oxalate ion must always be in considerable excess The reduction of the cerium(1V) ion5 in aqueous over the stoichiometric quantity. It has been noted sulfuric acid to the cerium(II1) ion with oxalate ion by the authors that as one approaches the stoichiocan be followed visually and without special colori- metric amount of oxalate, the time required for demetric devices. colorization becomes excessively long and deviates from the linear relationship shown by the more conHIO* ZCet4 + GO1-- + ZCeCS 2COp centrated solutions. (yellow-orange) (colorless) The authors have adapted this reaction to a laboraAt room temperature, the time required for complete tory experiment in general chemistry in connection with decolorization can he measured and related to the rate the topic of kinetics. Solutions of the reactants should be prepared and cooled prior to the laboratory period. BROWN, F. E., J. CHEM.EDUC., 25,256 (1948). A: "lfUric acid M. G., AND A. VISWANATHAN, J. CHEM.EDUC., SURYARAMAN,
+
28, 386 (1951). 'EVANS,G. G., J. CHEM.Eouc., 29, 139 (1952). BARRETI..R. L.. J. CHEM.EDUC.. , 32. 78 119551. ,~~ ~, 6 ~lthough'therebeems to be a great deal of evidence that the cerium(1V) ion exists ss a complex ion in aqueous sulfuric acid, this fact need not be introduced in the discussion of the react,ion in a beginning course in chemistry. ~
Solution B: 0.1 M ceric sulfate or ammonium sulfatocerate in 1 M sulfuric acid Solution C: 0.05 M sodium oxalate in 1 M sulfuric acid
~
solution A is used as a diluent in the preparation of the other solutions to maintain a constant acid concentration in all of the reaction rate measurements. Varia-
VOLUME 33, NO. 11, NOVEMBER, 1956
tion in acid concentration produces erratic result^.^ Solutions B and C should be standardized and the concentrations of each should be posted on the stock hottles. One liter of B and two liters of C are sufficient for a class of 30 working in pairs. EXPERIMENTAL PROCEDURE
A solution consisting of 10 ml. of B (celium(1V)) is diluted with 40 ml. of A in t,he reaction beaker. Twenty-five milliliters of C (oxalate) is diluted with 25 ml. of A in a second beaker. The time is noted by one of the students a t the instant that the other pours the oxalate solution into the reaction beaker containing the cerium(1V) and again when the latter decides that the solution is colorless. Thorough stirring during the reaction period is necessary. The disappearance of the yellow-orange color is best observed by placing the reaction beaker on a sheet of white paper next t,o a beaker of water. The timer and observer exchange duties and a duplicate run is made. The two observed time values are then averaged. A second pair of runs is made using the concentration of cerium(1V) ion just described but with solutions containing 20 ml. of the oxalate stock solution diluted with 30 ml. of Solution A. This is followed by a third set of runs using 15 ml. of C and 35 ml. of A. Temperature is measured after each run. The students are required to calculate the initial concentration of each of the react,ants in each of the Ross, S. D., (1947).
AND
C. G . SNAIN,J . Am. Chem. Sac., 69, 1325
three react,ion mixtures. The concentration of the oxalate ion in each mixture is then plotted against the time required for complete decolorisation. EXPERIMENTAL RESULTS
Five sets of results, chosen a t random from a class of 30 students, are given in the table. The data of Pair #2 would be classed as rather poor and represent an exception rat~herthan t,he rule. The data illustrate the typical variation to be expected because of differences in visual perception, lack of close t,emperature control, and the necessary lack of precision in volume measurements using only graduated cylinders. Time Required for Ceric Reduction by Oxalate (25' C.)Student Results
Initial cone. (molell.) -qf reactantsCe(1V) Oralate ion
ion
Tinw ,for decoloriration --solution Pair Pair Pair Pair Pair (see.) oj
#I
4
iY3
#L
4%
Time value reported for eaoh pair is an average of two trials.
The satisfaction obtained by the student when he performs his own experiment is frequently lost when a topic is introduced by way of demonstration. The authors feel that this experiment affords the student a rather simple yet satisfacto1.y method of studying the relationship of reaction rates t,o concentrations of reactants.
